Scaling, filtering, and scan conversion are common video post-processing functions in a decoder.
Scaling involves making an image or video frame larger or smaller than its original resolution. For example, a video frame may be scaled up to fit a large screen, scaled down to fit a small screen, or scaled to occupy only a portion of the screen (i.e., thumbnail display). During down scaling, the size of a frame is reduced by removing pixels. On the other hand, up scaling involves increasing the size of a frame by creating new pixels for interpolating between and/or replacing original pixels in the frame. Converting a video from NTSC (640×480) resolution to HDTV (1920×1080) resolution is an example of up scaling. An example of down scaling would be converting from HDTV to NTSC.
When a frame is scaled, the value of each new pixel in the scaled frame is based on the values of one or more pixels in the original frame. The calculation of each new pixel can be formulated as a linear equation in which the value of each original pixel used to calculate the new pixel is multiplied by a coefficient that determines the relative weight of the original pixel in the calculation. The value of a new pixel is typically calculated based its on neighboring original pixels, with different coefficients being used for different types of scaling. A frame may be scaled vertically and/or horizontally. In either case, the ratio of the number of output pixels to the number of input pixels is referred to as the scaling ratio. For a scaling ratio of m:n, there are m pixels in the scaled frame for every n input pixels from the original frame. For example, an up scaling of 33% can be represented by a ratio of 4:3.
Filtering involves modifying the pixels in a frame based on the desired characteristics of the output frame. For example, certain filters are used to generate a smoothed or blurred version of the original frame, while other filters might sharpen or enhance the edges that appear in the image. Contrast and brightness can also be adjusted using filtering operations. Filters can also be used to effect changes in color saturation. Filtering is similar to scaling in that the value of each filtered pixel is calculated based on a linear equation in which the values of one or more input pixels are multiplied by coefficients.
Scan conversion involves the conversion of the input video data stream from one format to another, such as from progressive scan format to interlaced scan format, or vice-versa.
Conventional solutions for performing scaling, filtering, and scan conversion typically involve the use of a large line buffer (SRAM), especially for high definition (HD) video applications. Such large line buffers are costly, as they increase chip size and power consumption. This is particularly problematic for a video processing system-on-chip (SOC) design, where small chip size and low power dissipation are highly desirable features.
In some conventional configurations, each of the scaling, filtering, and scan conversion functions are carried out in a corresponding pipeline stage. In such configurations, each stage requires its own line buffer (SRAM). Again, the use of such line buffers collectively occupies a significant amount of space and power consumption, and ultimately increases the cost of the chip or chip set. Also, using separate operations may adversely impact the operational speed.
What is needed, therefore, are techniques for performing scaling, filtering, and/or scan conversion that reduce the amount of line buffer space required in the overall design.